There are known a number of two-component developers comprising magnetic carriers and toner particles for use in various electrophotographic copying machines. For the image-development using such two-component developer in the electrophotographic copying machine, bristles of a magnetic brush are formed on the surface of a developing sleeve provided with a magnetic pole, and said magnetic brush is contacted with the electrostatic latent image formed on an electrophotographic photosensitive member to thereby form a toner image.
The use of ferrite particles as the magnetic carriers of the developer is well known. For example, Japanese Laid-Open Sho. 60(1985)-170863 discloses a two-component developer containing magnetic carriers comprising ferrite particles of 5.times.10.sup.7 .OMEGA..multidot.cm in specific resistance and of 50 to 120 .mu.m in mean particle size. The publication states that the two-component developer is effective to develop solid black areas with a uniform density without reducing resolution.
However, the two-component developer proposed by the foregoing Japanese publication is still problematic that it is not satisfactory in development of minute lines and dots. That is, for example, when an original containing multi-minute lines is reproduced with the use of said developer, the resulting image often becomes such that is not equivalent to the original since some of reproduced minute lines are wider or thinner than the original minute lines and accompanied by blank area. Further, Japanese Laid-Open Sho. 63(1988)-313174 discloses an electrophoto-graphic image-forming process wherein a toner image is formed by way of a so-called non-contact development using magnetic carrier particles each of which having a coat comprising an insulating material on the surface thereof and satisfying the equation: 30.ltoreq.M.ltoreq.-0.8R+150(10.ltoreq.R.ltoreq.150) wherein M represents a magnetic intensity (emu/cm.sup.3) which was measured in the magnetic field of 1000 oersted and R represents a mean particle size (.mu.m). This proposal is meaningful in the viewpoint that the existence of a specific interrelation between the magnetic intensity of the magnetic carrier and the particle size thereof. However, when the foregoing magnetic carrier particles are used in image-development of the electrophotographic image-forming process, it is difficult to match those magnetic particles with toner particles as desired. Thus, any of the foregoing proposals is directed to magnetic carrier particles for use in electrophotographic developers and those magnetic carrier particles are ones that have characteristics specified by the static condition but not by the dynamic condition which is necessary to be considered with respect to their characteristics upon contact of the developer's magnetic brush formed on the surface of the developing sleeve with the surface of the photbsensitive member.
Any of these known two-component developers is not sufficient to meet an increased demand for provision of an improved two-component developer which makes it possible to reproduce a desirable high quality image from an original containing multi-minute lines such as complicated chinese characters and black solid areas, which is not accompanied by any missing part and which excels in resolution and density (optical density).